Automatic gain control circuits



April 3, 1934.

P. o. FARNHAM 1,953,554

AUTOMATIC GAIN CONTROL CIRCUITS Filed lay 14. 1932 5 2 Sheets-Sheet 1 III April 3, 1934;

a. FARNH'AM AUTOMATIC GAIN CONTROL CIRCUITS 2 Sheets-Sheet I 2 Filed May 14, 1932 Patented Apr. 3, 1934 AUTOMATIC GAIN common omourrs Paul 0. Farnham, Boonton, N.- 3., assignor to Radio Frequency Laboratories, Incorporated, Boonton, N. .l., a corporation of New Jersey Application May it, 1932, Serial No. 611,402.

at Claims. (oi. 250-20) This invention relates to automatic gain control circuits of the general type now commonly employed in radio receivers, and more particularly to the so-called delayed action system in which the rectifier of the gain control system is heavily biased to postpone the development of an automatic gain control voltage until the received signal strength reaches that critical value which, at maximum amplifier gain, would produce the normal or desired voltage level at the reproducer.

While the delay action is desirable since it avoids loss of sensitivity for Weak signals, certain of these systems have been open to the objection that audio frequency distortion is introduced as the gain control rectifier comes into operation.-

For weak signals, the rectifier is inoperative and the voltage impressed upon the detector is an amplified reproduction of the received modulated carrier wave. As the peaks of the voltage wave rise above the delay bias on the rectifier, the form of the wave impressed upon the detector is distorted due to the cutting of the wave'peaks by the rectifier load.

Objects of the present invention are to provide methods of and circuit arrangements for automatically controlling the gain of an amplifier in such manner that the automatic gain control system does not distort the transmitted alternating current wave. More specifically, objectsare to provide methods of and circuit arrangements for postponing the development of an automatic gain control voltage in a transmission system, and which are characterized by rectifying one side of the modulated wave to obtain a gain control voltage and rectifying theother side to obtain an audio output.

These and other objects and advantages of the invention will be apparent from the following specification when taken with the accompanying drawings, in which Figs. 1 to 4, inclusive, are circuit diagrams of modulated carrier wave transmission systems embodying the invention.

In .the several views, the reference character SF identifies a radio frequency amplifier having input terminals A, G, the amplifier working into the rectifier of an automatic gain controlsystem and into a detector which has an audio frequency load indicated at AF. In the case of a radio re-- ceiver, the input terminals may be any convenient type of collector structure, and the audio frequency circuits include one or more amplifiers followed by a reproducer. So far as concerns the present inventiols'; the radio frequency and audio frequency amplifiers may be of any desired condenser C3.

design since the invention resides in the automatic gain control and detector circuits, and in the association of such novel circuits with am-= plifiers of'any appropriate design.

As shown in Fig. 1, the rectifier of the gain control system and the audio frequency rectifier or detector are combined in a single tube T and, while this arrangement is desirable from the standpoint of simplicity and economy, it will be apparent that two independent rectifier tubes may be employed. The tube T may be an ordinary triode or, as indicated diagrammatically may be a twin-grid tube having grids G1, G2, which cooperate with cathode K to form two diode rectifiers.

The amplified radio frequency voltage developed by the amplifier SF across its output coil L is impressed upon a tuned circuit which includes the inductances L1, L2 which are connected by a V, blocking condenser 01 and shunted by a tuning condenser C. The cathode is connected to the junction of one inductance and the condenser C1 by a lead which includes a radio frequency bypass condenser C2, and the outer ends of the inductances Ll, L2 are connected to the grids G1, G2, respectively.

The detector or audio frequency ,diode is constituted by grid G1 and cathode K, the audio frequency circuit including a resistance R1 which I 1 is connected between the cathode terminal of the 5 coil-L1 and ground. If desired, and as illustrated, the detector diode may be biased to prevent all audio output for weak signals by connecting the ground terminal of resistance R1 to a source of direct current potential which is negative with respect to the cathode K. This may be done conveniently by connecting the cathode K to a more positive point on the output resistance of a power supply unit, indicated by +B, than I the section B over which a tapped connection 35 from resistance R1 may be adjusted, the section B being shunted by a signal frequency by-pass The potential drop E1 across the portion of R included between the tap and the cathode K is thus applied as a negative bias on 1* grid G1 and suppresses rectification until the peak of the radio voltage exceeds the bias E1. The audio frequency potential developed across the resistance R1 is impressed, through condenser G4, on a voltage divider R2 and any desired portion of the available voltage may be impressed upon the audio load AF.

The output resistance R3 of the rectifier Gz- -K is connected between the cathode terminal of coil L2 and ground, and a delay bias voltage E2 is thus no gain control rectifier Gz-K does not distort the side of the modulated carrier wave which is impressed upon the detector diode.

When an ordinary triode, such as a type 27 tube, is employed in place of the double-grid tube, the control grid is connected to constitute the element G1 of Fig. 1 and the plate serves as the element G2.

The double diode tube T may be replaced, as

shown in Fig. 2, by a tube of the coplanar grid type T1. 'Tubes of this type include a cathode K, a plate P, and two grid systems which may be of any desired types, i. e., of triode, tetrode or pentode form. As shown, the tube T1 is of the triode type but it will be apparent that the invention is not limited to this particular form. The majority of thelcircuit elements may be identical with those of the Fig. 1 circuit and will not be described in detail but are identified by corresponding reference characters. vThe essen-. tial difference in the circuits is the connection of the resistance R5 between the plate P "and the B-stick to apply a direct current potential on the plate.

The presence of theplate P does not afiect the diode action of the gain control rectifier elements Gr-K.

A further embodiment of the invention is shown in Fig. 3, in which the tube 'T: is a special tube having two plates P1, P2 and two cathodes K1, K2 which are heated by a common filament F. The advantage of this tube over the single cathode type shown in Fig. l is that the full radio voltage E0 may be impressed on both cathodes and without losing the desired phase reversal, by connecting the cathodes to opposite sides of the same tuned input circuit L20; 2 With the appropriate changes necessitated by this use of independent. cathodes, the circuit elements and their arrangements are substantially the same as in Fig. 1. The audio system may be traced from plate. P1 through the input inductance L3, the tapped connection to the direct current source R. on the B-stick (when a delayed action is desired), a desired portion of resistance R, and the audio output resistance R1 which is connected to the cathode K1. The resistance R1 is by-passed for radio frequencies by condenser C2, and the audio output is passed through condenser C4 to the voltage divider R2.

The gain control rectifier circuit includes the connection of the cathode K: to that terminal of the coil La to which the plate P1 of the other E2 on the rectifier of thegain control system.

The. connection from the resistance R: to the control systems of the type in'which the rectifier and/or the detector elements of the transmission system are incorporated in a tube which has the additional function of an amplifier. Such an embodiment of the invention is shown in Fig. 4 in which the tubes T3, T4 have elements which permit operation as pentode amplifiers, and a second plate which cooperates with the cathode to form a diode rectifier.

The circuit elements associated with the pentode elements of the tube T3 provide a radio frequency amplifier stage following the amplifier SF, the input circuit comprising a tuned circuit LiC which is coupled between the control grid G1 and an intermediate point on the cathode bias resistor R6. The screen grid G2 is connected to a direct current source, indicated by +D and the suppressor grid G3 is connected to the cathode K, as is customary in a commercial type of pentode. The output circuit includes the coil L in the circuit of the pentode plate P and coupled to the coils L1, L2 which are in series with a blocking condenser C1, and shunted by a tuning condenser C.

The cathode K and plate P1 of the tube Ta form the diode rectifier of the automatic gain control I system, the outer terminal of the coil L2 being connected to the auxiliary plate P1 and the opposite terminal being connected to ground through the output resistance R3. The direct current voltage developed across the resistor R3 is applied, through the alternating current filter formed by resistance R4 and condenser C5, to the automatic gain control connection 1 of the amplifier, SF. The full potential drop across the cathode resistor R6 is impressed on the diode as adelay bias voltage which prevents the development of a direct current output until the carrier voltage peaks exceed the delay bias voltage.

The radio voltage across the coil L1 is applied to the elements.P1K of tube T4 by connecting the outer terminal of the coil to the plate P1, and connecting the inner terminal to ground through the audio frequency resistor R1 that is shunted by the radio frequency by-pass condenser C2. A tap on the resistor R1 provides a manual control for the output level by permitting a desired portion of the available audio frequency voltage to be applied to the control grid G1 of the tube T4, the connection including a radio frequency filter formed by resistance R; and condenser Ce. The pentode elements of tube T4 constitute an audio frequency amplifier which works into the audio load AF.

The coils L1, L: are wound in the same sense and it will be apparent that the phase of the radio voltage E across the diode rectifier of the tube Ta is opposite that of the voltage E across the diode rectifier of the tube T4. The distortion of one side of the amplified radio wave by the gain control rectifier does not, therefore, afi'ect the operation of the detector diode since that oper ates upon the other side of the radio wave.

It will be apparent that the invention is not limited to the particular circuits herein illustrated since, in particular, the combination of two substantially independent vacuum tubes in a single envelope is not an essential feature of the invention. It will be obvious, to those familiar with the design of transmission systems, that other circuits may be employed for obtaining the desired phase reversal in the radio voltages imsignal frequency amplifier SF is, as before pressed upon the gain control rectifier and upon through a resistance R4 and lead 1.

The invention may be incorporated in gain the detector.

I claim:

1. In a transmission system including an ampliiler and an automatic gain control, the method which comprises rectifying one side of the amplified radio frequency voltage to obtain an automatic gain control voltage, and rectifying the other side to obtain a modulation frequency output voltage.

2. In a transmission system including an amplifier and an automatic gain control of the delayed action type, the method which comprises initially biasing said rectifier to prevent rectification below a critical input level, rectifying one side of the amplified radio frequency voltage to obtain an automatic gain control voltage, and rectifying the other side to obtain a modulation frequency output voltage.

3. In the operation of a receiver including a rectifier for controlling the gain of an amplifier, the method of reducing audio distortion which comprises impressing upon the gain control rec= tifier and upon the detector radio frequency voltages of opposite phase.

4. In the operation of a receiver including an amplifier, and a rectifier for automatically developing a bias voltage for controlling the amplifier gain, the method which. comprises impressing the amplified radio voltage on a detector to obtain an audio frequency response, biasing the rectifier to prevent the development of an automatic gain control voltage for amplified radio voltages ne low a critical value, and impressing amplified radio'voltages of opposite phase on said rectifier and said detector.

5. A transmission system of the type including a radio frequency amplifier, means including a rectifier for automatically varying the amplifier gain with variations in the strength of a received radio wave, a detector, means biasing said rectifier to render the same inoperative for radio input voltages below a critical value which is in excess of the minimum value at which the detector becomes operative, and a network for impressing upon said rectifier and said detector radiofrequency voltages developed by said am plifier, characterized by the fact that said network includes means impressing radio frequency voltages of opposite phase upon said rectifier and detector.

6. A transmission system as claimed in claim 5, wherein said rectifier and said detector each comprises a diode.

'7. In a transmission system, the combination of a radio frequency amplifier, a gain control system including a rectifier for automatically controlling the amplifier gain to compensate for the varying strength of the received signals, a detector for demodulating the amplified radio frequency waves, said detector and said rectifier each having a high potential (radio frequency) input terminal and a common low potential terminal, and an impedance network coupling the radio frequency amplifier to said detector and said rectifier, said network comprising impedance elements coupled to said amplifier to impress outof-phase radio frequency voltages upon the respective high' potential terminals.

8. In a transmission system, the combination with a radio frequency amplifier, a detector, an audio frequency load, a rectifier upon which a radio voltage is impressed, and circuit elements for impressing the direct current output of said rectifier upon said amplifier as a gain control voltage, of means rendering the shape of the audio frequency wave developed by said detector independent of the rectifier action of said rectifier, said means comprising an impedance network coupled to said amplifier to impress one side of a radio wave upon said rectifier and to impress the opposite side thereof on said detector.

9. In a transmission system, the combination with a radio frequency amplifier, a detector, and

an audio frequency load, of a rectifier and asso ciated circuit elements for automatically rendering the radio voltage at a predetermined point in said amplifier substantially independent of fluctuations of the input radio voltage, and an impedance network coupling said detector and said rectifier to said amplifier, said network impressing upon said rectifier an amplified radio voltage opposite in phase to the radio voltage impressed on said detector. i

10. In a transmission system, the combination with a radio frequency amplifier, a rectifier and a detector working out of said amplifier, and means energized by said rectifier for automatically rendering the amplifier output substantially independent of fluctuations in the magnitude of received signals, of a network coupling said detector and said rectifier tosaid amplifier, said network comprising circuit elements reversing the phase of the amplified radio frequency voltages impressed on said rectifier and said detector.

11. The invention as set forth in claim 10, wherein said rectifier and said detector comprise elements located within a single evacuated envelope.

12. The invention as set forthin claim 10,,

wherein said rectifier and said detector comprise elements located within a single evacuated envelope and having a cathode in common, and said circuit elements include a tuned circuit of two serially connected inductances shunted by a tuning condenser, a connection for maintaining on the common cathode the radio frequency potential of the junction of said inductances, and connections from the outer terminals of said induce tanoes to the elements of said rectifier and said detector, respectively.

13. The invention as set forth in claim 10, wherein said rectifier and said detector are constituted by a single tube having a cathode and couplings between said amplifier and said detector and between the amplifier and said rectifier, said detector and rectifier being incorporated in a single tube having a pair of cathodes and a pair of anodes, of a common input circuit for said detector and said rectifier, said input circuit being oppositely connected to said rectifier and said detector.

15. In a transmission system the combination with a radio frequency amplifier, an automatic gain control system including a rectifier biased to prevent operation below a critical voltage, and audio frequency circuits including a detector and an audio frequency load, of input circuits for said rectifier and said detector, and means coupling said input circuits to said amplifier to impress one side of an amplified radio frequency wave on said detector and the opposite side thereof on said rectifier.

having a common input circuit, and the cathode of one diode and the anode of the other are connected to the same terminal of said common input circuit.

1'7. In a modulated signal wave transmission system, the combination with a multistage radio frequency amplifier, a tube in the final stage of said amplifier and having an auxiliary anode element cooperating with the tube. cathode to form a diode rectifier, and a detector, of a radio frequency output circuit for said tube, a coupling circuit for impressing on said detector and said rectifier the voltage output of said radio frequency amplifier, and means for, reversing the phase of the radio frequency voltages impressed on said rectifier and said detector..

18. The invention as set forth in claim 1'7, wherein said detector comprises the cathode and an auxiliary anode in a tube havingelements functioning as an audio frequency amplifier.

19. In a radio receiver, the combination with a radio amplifier, of a multiple function stage working out of said amplifier to develop an audio frequency voltage, said stage comprising a radio input circuit, an audio frequency output circuit,

' a direct current output circuit having a resistance included in a circuit for automatically controlling the gain of said amplifier, and a single tube having a cathode and an anode between which said radio input and said direct current output circuit are connected, and a second cathode and anode between which said audio frequency output circuit is connected.

2i]. In a radio receiver, a radio amplifier and a multiple function stage working out of said amplifier to develop an audio frequency potential for application to a succeeding load circuit and a direct current potential for automatically controlling the gain of said amplifier, said stage comprising a single tube housing a pair of cathodes and a plurality of anodes, a radio input circuit connected between one cathode and its anode, a resistance in an output circuit between said one cathode and its anode, means for returning to said amplifier as a gain control voltage the direct current potential developed across said resistance, and an audio frequency output circuit connected between the other cathode and its anode.

21. In a radio receiver, the combination with a radio amplifier, of a multiple function stage working out of said amplifier, said stage comprising a tube housing a cathode and an anode cooperating therewith to constitute a diode, a diode input circuit coupled to said amplifier, said circuit having its high potential (radio frequency) terminal connected to said cathode, a resistance between said cathode and anode, circuit elements including said resistance forming a biasing circuit for said amplifier, a second cathode and a second anode within said tube, an audio frequency output circuit between said second cathode and anode, and means cooperating with said second cathode and anode to develop an audio frequency voltage between the same.

22. In a radio receiver, the combination with aradio amplifier; of a multiple function stage comprising a single tube housing the anode element of a diode rectifier and the input element of adetector, cathode means cooperating with each of said elements; a tuned circuit coupled to said radio amplifier; connections from the terminals of said tuned circuit to the anode element and the input element, respectively; and means connecting said tuned circuit to said cathode means to establish a radio frequency voltage between the same and each of said elements; means impressing bias voltages of different magnitudes between said cathode means and the respective input elements; an output circuit between said anode element and said cathode means; and means including said output circuit for automatically controlling the transmission through said receiver as a function of the radio frequency input thereto,

23. In a radio receiver, the combination with a radio frequency amplifier, of a multiple function stage having a single tube housing a cathode and two anodes cooperating therewith to function as two diode rectifiers, means impressing a bias voltage of one magnitude on one anode and a bias voltage of a different magnitude on the other anode, input circuit means coupling said diode rectifiers to said amplifier, an output circuit for one diode rectifier including a resistance across which a direct current potential is developed, and means including said resistance for controlling transmission through said receiver; said input circuit means comprising a tuned circuit having inductance and capacity in parallel between the terminals thereof, a connection between said cathode and said tuned circuit to maintain said cathode at the radio potential at an intermediate point in said inductance, and connections from the opposite terminals of said tuned circuit to one and the other, respectively,

of said anodes.

24. In a radio receiver, the combination with a radio frequency amplifier, of a multiple tunetion stage having a single tube housing a cathode and two anodes cooperating therewith to function as two diode rectifiers, means impressing a bias voltage of one magnitude on one anode and a bias voltage of a different magnitude on the other anode, input circuit means coupling said diode rectifiers to said amplifier, an output circuit for one diode rectifier including a resistance across which a direct current potential is developed, and rneans'including said resistance for controlling transmission through said receiver; said input circuit means including an inductance in an output circuit of said amplifier, a pair of inductances coupled to said first inductance, and connections for impressing on one diode the radio voltage developed across one of the inductances of said pair and for impressing upon the other diode the radio voltage developed across the other of said pair of inductances.

25. In a radio receiver, the combination with a radio amplifier, of a multiple function stage including a single tube having a cathode cooperating with an anode to function as a diode rectifier and with an input electrode and a second anode to function as a demodulator system, a single tuned circuit coupled to said amplifier, connections between said circuit and certain of said tube elements to impress radio voltages across said diode rectifier and between said cathode and the input electrode of said demodulator system, means impressing bias voltages of different magnitudes between said cathode and respectively said first anode and said input electrode, means including said rectifier for control ling the gain of said radio amplifier, and an audio output circuit connected between said cathode and said second anode.

26. In a radio receiver, the combination with a radio amplifier, of a multiple function stage for developing audio and direct current voltages; said stage comprising a single tube housing cathwith the other input element and said cathode means to form a delayed diode rectifier system inoperative to develop-a direct current voltage when the impressed radio .voltage is of less than a predetermined critical value, and circuit connections for returning to said radio amplifier a gain control voltage which varies with the direct current voltage developed by said rectifier system.

PAUL O. FARNHAM. 

